In manually operated chucks some form of worm or scroll mechanism is often provided so that the chuck jaws tightly clamp on the outside diameter of a workpiece. In high production chucks this time loss in having an operator manually actuate the worm or scroll cannot be tolerated. Accordingly, most high production chucks are actuated through an axially movable actuator such as a draw bar from a fluid pressure source, either air or hydraulic. Some form of linkage changes this axial motion into radial motion to move the jaws inwardly and outwardly for chucking and releasing a workpiece. As chucks become larger in diameter and rotating speeds increase, the safety of operators of the machine becomes of far greater importance. The prior art has known of use of a check valve in the fluid line so that if fluid pressure fails the actuator hopefully does not move enough so that the chuck jaws would release the workpiece. The prior art has also known wedge type chucks in which the mechanical advantage is high enough, namely; the taper of the wedge is about 6 degrees, so that it acts as a locking taper and upon loss of fluid pressure the locking taper of the wedge prevents the outside diameter jaws from moving outwardly sufficiently to release the workpiece. The difficulty with the check valve arrangement is that it does not provide safety in case the threads are stripped on the draw bar or other mechanical parts in the actuating mechanism should fail. The difficulty with the locking taper on the wedge type chuck is that it takes an extra long range of movement of the actuator in order to provide sufficient range of radial movement of the chuck jaws. This long range of movement makes the chuck cumbersome and often weaker because of longer overhang of stressed parts.